what he means is this:
shoot the same scene twice. with a tripod so that the background does not move from shot to shot.
once with NOBODY in it, called the background plate
again with the "ghost" doing its thing in scene
Now imagine your timeline.
Bottom layer is your background plate. Which is video of a scene with nothing moving in it, or at least the camera isnt.
The layer which goes on top the that layer is the "ghost" video footage.
So you now have 2 layers of video, one with a person, one without. Both look exactly 100% the same except for the person in one of the videos.
If you were to turn the opacity of the 2nd layer, the ghost, down to say 50%, then you would be able to see thru the 2nd layer to the background plate.
because both layers contain the same video, one with a ghost, then the only thing that should look transparent is the person who is now ghostly.
The reason is that the 2nd layer is completely 50% transparent, but since the background plate and the 2nd layer match, except for the ghost, the only part that ends up being "see thru" is the person.
To change the opacity of a layer (or video) double click the footage so it opens in the viewer. Click the basic motion tab and set it for your transparency level. There is also the layer blending modes you can try as well.